The data capacity in the networks has been increasing year by year due to the wide spread of the Internet. In the trunk line connecting metropolitan areas, the optical transmission link whose transmission capacity per one channel is 10 Gb/s or 40 Gb/s has already been introduced. The On-Off-keying (OOK) is employed as a modulation scheme in 10 Gb/s transmission. Although the OOK scheme is also employed in 40 Gb/s transmission, it is unsuitable for long-haul transmission because the transmission characteristics are greatly influenced by the chromatic dispersion due to the narrow optical pulse width of 25 ps. Therefore, the multilevel modulation scheme using phase modulation and the polarization multiplexing scheme have been adopted, and the Dual Polarization Quadrature Phase Shift Keying (DP-QPSK) scheme is mainly employed for 100 Gb/s class transmission systems.
The optical signals modulated by DP-QPSK scheme in a transmitter are received and demodulated by a coherent optical receiver (refer to Non Patent Literature 1, for example). FIG. 12 shows an example of the configuration of a related coherent optical receiver. The related coherent optical receiver 600 has a local light source 610, a 90-degree hybrid circuit (90° HYBRID) 620, an optoelectronic converter (O/E) 630, an analog-to-digital converter (ADC) 640, and a digital signal processing unit (DSP) 650.
Signal light and local light can be represented by the following formulae as a single polarization signal, respectively.S(t)=exp[jωt]  (1)L(t)=exp[j(ω+Δω)t]  (2)
Here, Δω represents a frequency offset between the signal light and the local light. The signal light and the local light are inputted into the 90-degree hybrid circuit (90° HYBRID) 620, passing through an optical interference system, and converted into electric signals by the optoelectronic converters (O/E) 630, each of which is composed of differentially configured photodiodes. At this time, the outputs represented by the following formulae (3) and (4) are obtained from an Ix port and a QX port, respectively.IX(t)=cos(Δωt)  (3)QX(t)=sin(Δωt)  (4)
In the case of a polarization multiplexed signal, the signal light S (t) is expressed as S (t)=EX+EY, the cosine components of the mixed signal of EX+EY are outputted from IX port and IY port, and the sine components of the mixed signal of EX+EY are outputted from QX port and QY port.
The signals outputted from each port are AD converted by the analog-to-digital converters (ADC) 640, and then inputted into the digital signal processing unit (DSP) 650. The digital signal processing unit (DSP) 650 demultiplexes inputted signals to EX signal and EY signal by polarization demultiplexing processing, and demodulates the signals of EX and EY into four levels by phase estimation processing.
In this way, the DP-QPSK signals can be demodulated by using the coherent optical receiver.    Non Patent Literature 1: M. G. Taylor, “Coherent Detection Method Using DSP for Demodulation of Signal and Subsequent Equalization of Propagation Impairments”, IEEE Photonics Technology Letters, vol. 16, No. 2, February 2004, p. 674-676